Titanium is the second most abundant transition metal and is already a key player in important industrial processes (e.g. polyethylene). Titanium is an attractive metal to use for catalytic transformations as it is a versatile and inexpensive metal of low-toxicity and of established biocompatibility. However, its potential use as a catalyst for the synthesis of fine chemicals, pharmaceuticals and agrochemicals is often overlooked due to its oxophilic, Lewis acidic character, which renders complexes of titanium less functional group tolerant than their late transition metal counterparts. Nevertheless, three different fields of research in titanium catalysis have drawn attention in recent years: formal redox catalysis, hydroamination and hydroaminoalkylation. For these reactions, titanium offers new approaches and alternative pathways/mechanisms that are complementary to late transition metal-based catalysis. This review focuses on advances in fine chemical synthesis by titanium-catalyzed reactions featuring redox transformations and two important hydrofunctionalization reactions, hydroamination and hydroaminoalkylation. Starting from the late 90s, we provide an overview of historic inspirational contributions, both catalytic and stoichiometric, and the latest insights in catalyst design efforts, mechanistic details and utility of the three different classes of transformations. Insights to enhance catalyst activity as well as catalyst controlled regio- and stereoselectivities are presented. Illustrative examples that highlight substrate scope and the application of titanium catalysis to the synthesis of complex organic small molecules, natural products and materials are shown. Finally, opportunities and strategies for on-going research and development activities in titanium catalysis are highlighted.

Titanium catalysis for the synthesis of fine chemicals – development and trends

Synthesis of Isoxazolines and Oxazines by Electrochemical Intermolecular [2 + 1 + n] Annulation: Diazo Compounds Act as Radical Acceptors.

Active species for synthetic and catalytic applications are formed from well defined complexes or mixtures of compounds. For group 4 metallocenes, three pathways for the formation of the reactive complex fragment [Cp'2M] are known: (i) reductive mixtures and well defined complexes which are able to form the metallocene fragments either by (ii) addition or (iii) substitution reactions. In this account for each of theses systems (i)-(iii) a prominent example will be discussed in detail, (i) the Negishi reagent Cp2ZrCl2/n-BuLi, (ii) bis(η5 : η1-pentafulvene) complexes and (iii) metallocene bis(trimethylsilyl)acetylene complexes, to show the advantages and the disadvantages for each of these methods for synthetic applications. This account summarizes some main advantages of group 4 metallocene bis(trimethylsilyl)acetylene complexes as metallocene generating agents over other synthetically used systems. For each of the special purposes, all described systems have advantages as well as disadvantages. The aim of this overview is to help synthetic chemists in selecting the most effective system on the basis of [Cp'2M] (M=Ti, Zr) for synthetic or catalytic puposes.

Advantages of Group 4 Metallocene Bis(trimethylsilyl)acetylene Complexes as Metallocene Sources Towards Other Synthetically used Systems.

Oxo/imido heterometathesis: From molecular stoichiometric studies to well-defined heterogeneous catalysts

Reactivity Studies of a Bis(π‑η 5 :σ‑η 1 ‑benzofulvene)titaniumComplex Including Simultaneous N–H and C(sp 2 )–HActivation of Dibenzylamine

Direct Access to Titanocene Imides via Bis(η5:η1-penta-fulvene)titanium Complexes and Primary Amines

Transition-metal carbene complexes and their reactivities are a key topic of chemistry. They are an integral part of researches in catalysis, organic synthesis, coordination chemistry, and numerous other areas. In this context, we report the synthesis of a low-valent bis(η5 :η1 -(di-p-tolyl)-pentafulvene)niobium chloride. Owing to the π-η5 :σ-η1 coordination mode of the pentafulvenes and the resulting high nucleophilic character of the exocyclic carbon atom of the ligand, the bis(η5 :η1 -pentafulvene)niobium complex is able to achieve the umpolung of a coordinated vinyl unit and the resulting formation of the first η5 :η1 cyclic niobium Schrock carbene complex. This new synthetic route is, in comparison to classical α-hydrogen elimination reactions or thermolysis of diazo compounds, completely unprecedented. The reactivity of the cyclic carbene function and the remaining fulvene ligand is demonstrated by double N-H bond activation of primary amines to niobium imido complexes.

Bis(η5 :η1 -pentafulvene)niobium(V) Complexes: Efficient Synthons for Niobium Carbene and Imido Derivatives.

Cooperative Reactions of Pentafulvene Niobium Complexes: Formation of Alkylidene, Imido, Hydrazido, and Niobaaziridine Complexes

The π-η5:σ-η1 coordination mode of early transition metal pentafulvene ligands yields a strongly nucleophilic exocyclic carbon atom (Cexo). The substitution of the chlorido ligand of bis(η5:η1-(di-p-tolyl)pentafulvene)niobium chloride (1) by reaction with ethyl magnesium bromide is subsequently followed by a β-C-H activation employing this Cexo, forming the pentafulvene niobium ethylene complex 2. The intermediately formed ethyl complex can be intercepted with water, protonating both pentafulvene moieties and thereby retaining the ethyl moiety to give the terminal oxo complex 3. Complex 2 shows cooperative reactions of the remaining pentafulvene and the ethylene ligand. While the pentafulvene functions as a proton acceptor, the ethylene can be liberated to provide a NbIII metal center, available for E-H bond addition. Thereby, the imido hydride complex 4 and niobaaziridine hydride 5 are obtained. These hydride complex formations are investigated by deuterium labeling, concluding the redox mechanism. An alternative β-hydride elimination pathway is further disproven by purposely synthesizing a proposed intermediate and thermally treating it, to show that it does not undergo the β-hydride elimination under reaction conditions.

/pdf/a-niobium-pentafulvene-ethylene-complex-synthesis-properties-3tvv4fep.pdf

A niobium pentafulvene ethylene complex: synthesis, properties and reaction pathways.

Niobium Bis‐ and Mono(Pentafulvene) Complexes: E–H Bond Cleavage and Insertion Reactions (E = C, N, O)

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